World's first underwater nuclear explosion

Operation Crossroads was a series of nuclear weapon tests conducted by
the United States at Bikini Atoll in mid-1946. It was the first test of a
nuclear weapon after the Trinity nuclear test in July 1945, and the
first detonation of any nuclear device following the Fat Man detonation
on August 9, 1945. Its purpose was to investigate the effect of nuclear
weapons on naval ships.

Mushroom-shaped cloud and water column from the underwater Baker nuclear explosion of July 25, 1946. Photo taken from a tower on Bikini Island, 3.5 mi (5.6 km) away. (Image courtesy of US Govt. Defense Threat Reduction Agency.)

Mushroom-shaped cloud and water column from the underwater Baker nuclear explosion of July 25, 1946. Photo taken from a tower on Bikini Island, 3.5 mi (5.6 km) away. (Image courtesy of US Govt. Defense Threat Reduction Agency.)